1. Field of the Invention
The present invention is directed to a composite sheet material useful as a component for asphalt shingles, which provides shingles having improved tear strength, without compromise of tensile and flextural strength.
2. Description of the Prior Art
High strength, uniform thin sheets or mats of glass fibers have become very important in the building materials industry. Probably the best example of the use of this type of material is in roofing shingles. The art is replete with descriptions of glass fiber mats and methods of making those mats having improved strength characteristics formed of glass fibers and made commercially by a wet-laid process.
An interesting description of the development of this process is set forth in U.S. Pat. No. 4,135,029. Glass fiber mats made by the wet-laid process are formed by combining glass fibers held together by a binder material. Although binders useful in this application include urea-formaldehyde resins, phenolic resins, bone glue, polyvinyl alcohols, acrylic resins and polyvinyl acetates, urea-formaldehyde resins are preferred due to their low cost.
Earlier developments of glass fiber mats focused upon improvement in tensile strength. For example, U.S. Pat. No. 4,178,203 describes the addition of an anionic surfactant having at least one hydrophobic segment containing from 8 to 50 carbon atoms and an anionic segment which may be carboxy, sulfate ester, phosphate ester, sulfonic acid or phosphonic acid. Alternatively, the anionic surfactant may be a soap selected from a sodium, a potassium, an ammonium and an alkylammonium salt of a C10-C20 fatty acid.
U.S. Pat. No. 4,430,158 provides improved tensile strength to a sized glass fiber mat by adding an anionic surfactant which contains hydrophobic segments containing from 8 to 30 carbon atoms and anionic segments which may be carboxy, sulfate ester, phosphate ester, sulfonic acid and phosphonic acid.
Yet a further means of improving tensile strength of glass fiber mats employed as roofing shingles is taught in U.S. Pat. No. 4,542,068 which discloses a method of making a glass fiber mat in which an alkoxylated alkylamine having the formula
is added to a binder composition which comprises urea-formaldehyde and in which glass fibers are dispersed in a wet-laid process.
Although these and other methods have been devised for improving tensile strength of glass mat fibers, these improvements do not address a significant problem associated with the use of glass mats employed in roof shingles.
Those skilled in the art are aware that a major fabrication difficulty in the production of roofing shingles using glass fibers mats is meeting the ASTM standard for tear resistance, which is required for ASTM certification. Oftentimes, means utilized to increase tensile strength of glass fiber mats, for example, the addition of latex, specifically a styrene-butadiene latex copolymer, as described in U.S. Pat. No. 4,917,764, result in reduced tear strength of shingles made from such mat.
The tear resistance is the force required to rip a sample of material having a standard geometry. Roofing shingles are tested for tear resistance in accordance with ASTM Standard Test Procedure D 1922. This test involves the use of an Elmendorf apparatus. In certain applications, roofing shingles must conform to ASTM Standard D 3462, which requires a tear strength of 16.7 N (1704 grams force (gf)). Ordinary roofing shingles often fall short of this minimum tear strength.
The art has previously overcome this deficiency in tear strength by raising the weight of the glass mat and the asphalt disposed thereon. However, this expedient is costly.
The above remarks establish the need in the art for a new composite glass fiber mat sheet utilizable as a component of a roofing shingle, and a method of preparing that composite sheet, which provides shingles having improved tear resistance without seriously adversely affecting tensile strength.